EP0651565B1 - Circuit for compensating the drift of the level of the direct current of a video signal - Google Patents

Circuit for compensating the drift of the level of the direct current of a video signal Download PDF

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Publication number
EP0651565B1
EP0651565B1 EP94307998A EP94307998A EP0651565B1 EP 0651565 B1 EP0651565 B1 EP 0651565B1 EP 94307998 A EP94307998 A EP 94307998A EP 94307998 A EP94307998 A EP 94307998A EP 0651565 B1 EP0651565 B1 EP 0651565B1
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EP
European Patent Office
Prior art keywords
signal
video signal
level
circuit
synchronization signal
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
EP94307998A
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German (de)
English (en)
French (fr)
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EP0651565A1 (en
Inventor
Tomohide Matsuo
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Acer Inc
Original Assignee
NEC Corp
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Filing date
Publication date
Application filed by NEC Corp filed Critical NEC Corp
Publication of EP0651565A1 publication Critical patent/EP0651565A1/en
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Publication of EP0651565B1 publication Critical patent/EP0651565B1/en
Anticipated expiration legal-status Critical
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N5/00—Details of television systems
    • H04N5/14—Picture signal circuitry for video frequency region
    • H04N5/16—Circuitry for reinsertion of dc and slowly varying components of signal; Circuitry for preservation of black or white level
    • H04N5/18—Circuitry for reinsertion of dc and slowly varying components of signal; Circuitry for preservation of black or white level by means of "clamp" circuit operated by switching circuit
    • H04N5/185—Circuitry for reinsertion of dc and slowly varying components of signal; Circuitry for preservation of black or white level by means of "clamp" circuit operated by switching circuit for the black level
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N5/00—Details of television systems
    • H04N5/14—Picture signal circuitry for video frequency region
    • H04N5/16—Circuitry for reinsertion of dc and slowly varying components of signal; Circuitry for preservation of black or white level
    • H04N5/18—Circuitry for reinsertion of dc and slowly varying components of signal; Circuitry for preservation of black or white level by means of "clamp" circuit operated by switching circuit

Definitions

  • This invention relates to a sag compensation circuit for a video signal for compensating a sag of the video signal caused by loss of a low frequency component of the video signal.
  • an input video signal includes a sag component
  • a luminance gradient which is not included in the original picture, appears in a reproduced video signal on a display or the like and results in a degradation of picture quality.
  • the sag In order to improve the picture quality, the sag must be compensated.
  • Fig. 1 shows a block diagram of a first conventional sag compensation circuit.
  • the first conventional circuit consists of:
  • the clamp circuit 2 includes:
  • the synchronization signal generator 3 generates the generated synchronization signal which synchronizes with the synchronization signal of the input video signal.
  • the clamp pulse generator 22 In response to the generated synchronization signal, the clamp pulse generator 22 generates the clamp pulse signal which causes the switch 22 to close at the tip of the synchronization signal of the input video signal.
  • a predetermined portion of an output video signal 8 (for example, a tip of a synchronization signal of the output video signal) is clamped, or level-shifted to near the reference voltage of the reference voltage generator 24.
  • the first conventional sag compensation circuit has a problem that a small residual sag component remains in the output video signal. This is because a resistance component of the switch 22 prevents a tip level of a synchronization signal of the output video signal from being set to the reference voltage accurately.
  • Fig. 2 shows a block diagram of a second conventional sag compensation circuit, which is described in JP-A-62-272766 (1987).
  • the second conventional circuit includes:
  • the input video signal 1 is added to an output of the DC amplifier 98 in the adder 95, an output which is inverted and DC-amplified in the DC inverting amplifier 96 in order to output the output video signal 8.
  • the output video signal is also supplied to the sync-tip detector 97.
  • the sync tip detector detects a DC level of a tip of a synchronization signal of the output video signal and holds the DC level until the arrival of the next tip.
  • the DC amplifier amplifies the DC level and supplies the amplified signal to the adder 95.
  • the amplified signal of DC amplifier 98 decreases and vice versa, because the DC inverting amplifier 96 and the DC amplifier 98 have a different polarity of amplification relative to each other.
  • the sag in the input video signal can be compensated.
  • the second conventional circuit overcomes the problem of the first conventional circuit.
  • the second conventional circuit has the following new problem.
  • the impulse noise appears in the adder 95 output and circulates in the closed loop of Fig. 2. This circulation degrades the picture quality. Therefore, the second conventional circuit does not accurately compensate for the sag when the input video signal is noisy.
  • US-A-5008,753 describes a clamp system for a television signal, the system comprising a first clamp device for clamping an input television signal to a clamp control voltage supplied from an external source by a horizontal clamp pulse and a second clamp device for controlling the DC potential of the television signal by adding the externally-supplied control voltage to an input television signal.
  • the first clamp device is used during the period requiring an early clamp start, and the second clamp device is operated under normal conditions.
  • a sag compensation circuit for a video signal each scanning line of which includes a synchronization signal portion and an active video signal portion, said circuit comprising:
  • the preferred embodiment of the invention comprises:
  • a tip or a pedestal of a synchronization signal of a video signal may be used.
  • FIGs. 4A to 4H show timing charts for explaining the operations of the preferred embodiment.
  • the input video signal 1 is supplied to the first clamp circuit 2 and to the synchronization signal generator 3. Since a combination of the first clamp circuit 2 and the synchronization signal separator is the same as the first conventional sag compensation circuit of Fig. 1, a residual sag component signal appears in the clamped video signal as shown in Fig. 4A.
  • the level detector 4 detects the level of the or a pedestal of the clamped video signal (Fig. 4A) in accordance with the generated synchronization signal, which is shown in Fig. 4E.
  • the detected level at each tip of the clamped video signal is shown in Fig. 4B.
  • a variation of the detected level indicates the residual sag component.
  • the detected level is supplied to the converter 5.
  • the converter 5 converts the detected level to the reference level signal so that the greater the detected level becomes, the smaller the reference level signal becomes.
  • Fig. 4C indicates the reference level signal.
  • the first delay circuit 6a delays the clamped video signal by T 1 and outputs the delayed clamped video signal, which is shown in Fig. 4D, to the second clamp circuit 7.
  • the second delay circuit 6b delays the generated synchronization signal by T 1 and outputs the delayed synchronization signal, which is shown in Fig. 4F, to a capacitor 71 of the second clamp circuit 7.
  • the time T 1 corresponds to a sum of processing delays of the level detector 4 and converter 5.
  • the delay circuits 6a and 6b are provided in order to compensate for the sum of the processing delays. Therefore, if the sum is negligibly small, the delay circuits 6a and 6b may be omitted.
  • An internal embodiment of the second clamp circuit 7 is substantially the same as that of the first clamp circuit 2, except that the reference level signal is supplied to a second terminal of a switch 73 instead of a provision of a reference voltage source (RVS).
  • RVS reference voltage source
  • the delayed clamped video signal is supplied to a capacitor 71 and a clamp pulse generator 72 generates a clamp pulse signal (Fig. 4G), which causes the switch 43 to close when a tip of the delayed synchronization signal is inputted therein. Therefore, the second clamp circuit clamps a tip of a synchronization signal of the delayed clamped video signal to the reference level signal.
  • the sag included in the input video signal is fully compensated at an output of the second clamp circuit 7 and the second clamp circuit 7 outputs an output video signal which is free from sag as shown in Fig. 4H.
  • Fig. 5 shows a block diagram of the level detector 4.
  • the level detector comprises: an analog-to-digital (A/D) converter 41 for digitizing the clamped video signal from the first clamp circuit 2; a clock generator 43 for generating a clock signal for the A/D converter 41 and a latch pulse signal for a latch circuit 42; and the latch circuit which holds the value of an output of the A/D converter 41 at the time identified by the latch pulse signal.
  • A/D analog-to-digital
  • the clock generator In response to the delayed synchronization signal, the clock generator generates the clock signal for A/D converter 41 and a latch pulse signal for the latch circuit 42.
  • the A/D converter 41 converts the clamped video signal from the first clamp circuit 2 to digital data in accordance with the clock signal and supplies the digital data to the latch circuit 42.
  • the latch circuit holds a level of a tip of a synchronization signal of the clamped video signal until the next arrival of the next tip, as shown in Fig. 4B.
  • a signal corresponding to the level of the tip, or the detected level is supplied to the converter 5 of Fig. 3.
  • the converter 5 includes: a data converter 51 for converting the detected level to converted data which decreases when the detected level increases, and vice versa; a digital-to-analog (D/A) converter 52 for converting the converted data to the reference level signal; and a buffer amplifier 53 with low output impedance through which the reference level signal is supplied to the second clamp circuit 7 of Fig. 3.
  • D/A digital-to-analog
  • the data converter may be constituted by a Read Only Memory (ROM) or the like. Each address of the ROM corresponds to each value of the detected and the corresponding value of the each value of the detected level and the corresponding value of the reference level signal is stored in each address.
  • An example of the contents of the data converter 51 is shown in Fig. 7.
  • the data converter converts the detected level to the converted data, which is supplied to the D/A converter 52 and is converted to an analog signal, or the reference level signal.
  • the reference level signal is supplied to the second clamp circuit 7 via the buffer amplifier 53.
  • a quantity indicating a sag is obtained by sampling a tip level of a synchronization signal included in a video signal, it is also obtained by sampling a pedestal level of a synchronization signal of a video signal.
  • the first and the second clamp circuit may clamp a pedestal of a video signal inputted therein to the reference voltage and the reference level signal, respectively.
  • the present invention provides a sag compensation circuit which compensates a sag included in an input video signal accurately even when the input video signal is noisy.

Landscapes

  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Picture Signal Circuits (AREA)
EP94307998A 1993-11-02 1994-10-31 Circuit for compensating the drift of the level of the direct current of a video signal Expired - Lifetime EP0651565B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP273925/93 1993-11-02
JP5273925A JPH07131677A (ja) 1993-11-02 1993-11-02 ビデオ信号のサグ補正回路

Publications (2)

Publication Number Publication Date
EP0651565A1 EP0651565A1 (en) 1995-05-03
EP0651565B1 true EP0651565B1 (en) 1998-12-23

Family

ID=17534487

Family Applications (1)

Application Number Title Priority Date Filing Date
EP94307998A Expired - Lifetime EP0651565B1 (en) 1993-11-02 1994-10-31 Circuit for compensating the drift of the level of the direct current of a video signal

Country Status (4)

Country Link
US (1) US5508749A (ja)
EP (1) EP0651565B1 (ja)
JP (1) JPH07131677A (ja)
DE (1) DE69415479T2 (ja)

Families Citing this family (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2003324627A (ja) * 2002-04-26 2003-11-14 Matsushita Electric Ind Co Ltd 同期信号重畳装置及び同期信号重畳方法
US7023497B2 (en) * 2002-07-31 2006-04-04 Texas Instruments Incorporated Clamping circuit with wide input dynamic range for video or other AC coupled signals
JP2004088406A (ja) * 2002-08-27 2004-03-18 Sanyo Electric Co Ltd 画像信号処理装置
JP2004215184A (ja) * 2003-01-08 2004-07-29 Renesas Technology Corp クランプ回路
US7345714B2 (en) * 2005-01-13 2008-03-18 National Semiconductor Corporation Video signal clamp
JP5169861B2 (ja) * 2009-01-19 2013-03-27 ミツミ電機株式会社 半導体集積回路、映像信号出力回路

Family Cites Families (21)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS57131175A (en) * 1981-02-06 1982-08-13 Nec Corp Video signal clamping circuit
US4414572A (en) * 1982-03-15 1983-11-08 Rca Corporation Clamp for line-alternate signals
JPS59134967U (ja) * 1983-02-25 1984-09-08 ソニー株式会社 撮像装置
JPS59218080A (ja) * 1983-05-25 1984-12-08 Matsushita Electric Ind Co Ltd ペデスタルレベルクランプ回路
US4626910A (en) * 1983-12-13 1986-12-02 Fuji Photo Film Co., Ltd. Circuit for preventing flicker attributable to field signal-frame signal conversion
JPS615686A (ja) * 1984-06-20 1986-01-11 Sanyo Electric Co Ltd ビデオａｇｃ回路
JPS61140279A (ja) * 1984-12-12 1986-06-27 Toshiba Corp 映像信号処理回路
US4707741A (en) * 1986-04-11 1987-11-17 Harris Corporation Video signal clamping with clamp pulse width variation with noise
JPS62272766A (ja) * 1986-05-21 1987-11-26 Canon Inc ビデオ信号処理装置
JPH074002B2 (ja) * 1986-08-20 1995-01-18 松下電器産業株式会社 テレビジヨン信号クランプ装置
KR900006464B1 (ko) * 1987-05-23 1990-08-31 삼성전자 주식회사 버어스트 게이트 펄스를 출력 할 수 있는 동기신호 분리 집적회로
JPS6441576A (en) * 1987-08-07 1989-02-13 Nec Corp Clamping circuit
JPH0210970A (ja) * 1988-06-28 1990-01-16 Fujitsu Ltd クランプ回路
JPH02250477A (ja) * 1989-03-23 1990-10-08 Victor Co Of Japan Ltd ビデオ信号処理回路
JPH0323778A (ja) * 1989-06-20 1991-01-31 Mitsubishi Electric Corp 映像信号クランプ回路
US5128764A (en) * 1989-10-27 1992-07-07 Siemens Aktiengesellschaft Level correcting circuit having switched stages of differing time constants
JPH0771290B2 (ja) * 1989-12-27 1995-07-31 富士写真光機株式会社 信号処理回路
JPH03206774A (ja) * 1990-01-08 1991-09-10 Matsushita Electric Ind Co Ltd 画像処理装置
JPH0496476A (ja) * 1990-08-09 1992-03-27 Matsushita Electric Ind Co Ltd クランプ回路
JPH05284384A (ja) * 1992-03-31 1993-10-29 Nec Corp 映像信号用クランプ回路
FR2690032B1 (fr) * 1992-04-14 1996-02-23 Thomson Lgt Procede d'alignement en television et dispositif pour sa mise en óoeuvre.

Also Published As

Publication number Publication date
JPH07131677A (ja) 1995-05-19
DE69415479T2 (de) 1999-05-12
DE69415479D1 (de) 1999-02-04
US5508749A (en) 1996-04-16
EP0651565A1 (en) 1995-05-03

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